Project Summary Based on the gate-control theory of pain, traditional spinal cord stimulation (SCS) to treat chronic back/leg pain utilizes 40-60 Hz stimulation that activates spinal dorsal columns to elicit paresthesia over a patient?s painful region. This paresthesia-based SCS is only effective for 40-50% of patients with chronic back/leg pain, and the efficacy gradually reduces over time. A recent advance in SCS employs a high-frequency (10 kHz) biphasic stimulation waveform (HF10-SCS) at a subthreshold intensity that is paresthesia-free. HF10-SCS is effective for 80-90% of patients with a better and more sustained long-term (24 months) efficacy than traditional SCS. The superiority of HF10-SCS over traditional SCS and the paresthesia-free feature indicate that a mechanism different from the gate-control theory of pain is probably involved in HF10-SCS. Since it is well known that high-frequency (kHz) biphasic stimulation (HFBS) can block axonal conduction, previous studies have suggested that HF10-SCS blocks axons in the dorsal roots or axons/neurons in the spinal cord. However, recent computer modeling and animal studies suggest that paresthesia-free, low intensity HF10-SCS is not strong enough to either activate or block the axons in the spinal cord. To resolve these conflicting hypotheses, we need to first understand the mechanisms underlying HFBS block of a single axon. Unfortunately, how HFBS blocks a single axon is currently unknown. Therefore, in this grant application we will focus on revealing the mechanisms of HFBS block of single axons. We hypothesize that there are two types of HFBS nerve bock: 1. acute nerve block that occurs only during HFBS; 2. post-stimulation block that occurs during and after HFBS. Although the acute nerve block requires a supra-threshold HFBS, the post-stimulation nerve block can be induced by HFBS at a sub-threshold intensity without producing paresthesia. Understanding how HFBS blocks a single axon is critical for further understanding the mechanisms underlying HF10-SCS suppression of pain. We propose to combine modeling analysis and animal experiments to reveal the changes in ion gradients, ion channels, and ion pumps that underlie HFBS axonal block and the recovery of conduction following the block. We will reveal the biophysics underlying HFBS at the axonal membrane ion channel level by systematically characterizing, modeling, and validating the axonal response/block induced by HFBS. The knowledge acquired from our studies is important for understanding neural response to HFBS at a single axon level either in the central nervous system (spinal cord or brain) or in the peripheral nervous system. Our project is significant for public health because it provides the basic knowledge not only for understanding the clinically- proven efficacy of HF10-SCS therapy but also for developing new therapies employing HFBS in the central or peripheral nervous system.